Re: "On The Wagon"

: I vaguely remember a definition of the phrase being described by a London Taxi Driver on a PBS special several years ago. The taxi driver was driving the streets of London and speaking with the interviewer when something he passed triggered an explanation (my memory of what triggered the discussion is less clear) of the phrase "On The Wagon". He said that when prisoners were being escorted, by wagon (in much older times) to where they were to be executed the guards would stop at a pub and give them a final drink. After their drink the bartender would ask if they wanted another and the guards would say "No, he's On The Wagon". Has anyone heard this before? or can this PBS story be dug up for verification?

What I was told by a tour guide in London was that the condemned were taken on the wagon to Tyburn. He would be offered "one for the road" by his friends, while the driver who was prohibited from drinking was "on the wagon". A neat explanation of two expressions, but I'm not sure about the accuracy!